This invention relates to polymers having nonlinear optical properties, and is particularly concerned with main-chain chromophoric polymers with second-order nonlinear optical properties.
The field of nonlinear optics and electro-optics is concerned with the interactions of electromagnetic fields with media to produce new fields that are altered in phase, frequency, amplitude or other propagation characteristics from the incident field The best known nonlinear optical (NLO) effect is second harmonic generation (SHG) or frequency doubling. Optically nonlinear materials are used in frequency doublers for lasers, optical communications and computing equipment, laser resistant surfaces, and in opto-electronic devices for other applications.
Materials with second-order nonlinear optical properties include inorganic, organometallic, and organic crystals, which show good second-order nonlinear optics but it would be extremely difficult to use these materials as waveguides.
Nonlinear optical devices such as frequency doublers have been based almost exclusively on crystalline inorganic materials such as lithium niobate and potassium dihydrogen phosphate (KDP). The disadvantages of these materials include relatively slow response times to optical signals, poor laser damage resistance, relatively large dielectric constants, and difficulties in fabrication into opto-electronic devices.
Organic polymeric materials with large delocalized pi-electron systems exhibit very fast NLO responses, have large optical nonlinearities, low dielectric constants, large electro-optic coefficients, and the chemical synthesis of these material can be altered to optimize their desirable physical characteristics while preserving their NLO properties.
Polymeric NLO materials can have very good mechanical properties. They can be mechanically tough and easily fabricated or processed into thin film geometries that are very desirable for coatings and integration with microelectronics.
Two basic approaches exist for the synthesis of optically nonlinear polymers. One approach is to prepare guest-host materials by simply dissolving polarizable moieties (chromophores or dyes as the guest) in a polymeric host. This physical or solid solution may be severely limited in concentration of the dye due to solubility of the chromophore. Another disadvantage is the slow relaxation of dipole alignment with time.
The other approach is to synthesize polymers that have chromophores chemically attached as pendant side-chain substitutes. These dye-substituted polymers have several distinct advantages over guest-host materials including higher limiting concentrations of the chromophore, reduced mobility and enhanced orientational stability of the chromophore, and improved optical, thermal and mechanical properties.
Of all the previously known materials, perhaps the most promising are the main-chain chromophoric polymers having the chromophore chemically bonded to the polymer chain at two sites. These systems show good transmittance, stability and second-order optical properties.
In the article "Nonlinear Optical Properties of Polymeric Materials" by C. S. Willand et al., Ber Bunsenges, Phy. Chem. 91, pp. 1304-1310 (1987), nonlinear optical effects in main-chain systems such as copolymers derived from p-oxy-cyanocinnamate derivatives, are discussed. However, the low glass transition temperature of this copolymer makes it impossible to fabricate a stable, noncentrosymmetric film. In the more recent article "Head-to-Tail Assemblies of Dipolar, Piperazine-Linked Chromophores" by H. E. Katz, et al., J. Am. Chem. Soc. III, 7554-7557 (1989), a main-chain chromophoric oligomer has been reported. However, it is difficult to dissolve, and its fabrication into a nonlinear optical film has not been disclosed.
One object of the invention accordingly is the provision of novel polymers having nonlinear optical properties.
Another object is to provide main-chain chromophoric polymers having second-order nonlinear optical properties and a glass transition temperature (T.sub.g) substantially above room temperature.
Another object is to provide main-chain chromophoric polymers having electro-optic properties and a glass transition temperature above 100.degree. C.
Still another object is to provide optical films which ca be patterned by photoreaction techniques which may be useful holographic materials.
Yet another particular object is the provision of novel polymers derived from phenylene, phenylene vinylene, stilbenylene, phenyl piperdine, and coumarin, having the above characteristics.